The Master Hand is the Boss of Super Smash Bros.'s single player mode and Super Smash Bros. Melee's classic mode. The Master Hand has the apperance of a giant, floating, white, disembodied glove that attacks you on a plain dramatic area (Final Destination for example). He is the creator of Giga Bowser. His counterpart is Crazy Hand.
In the original game Master Hand was metaphorically depicted as a human (a child perhaps) who plays with the Smash bros. fighters which were toys. This theme of toy vs. human was abandoned however in Melee for a more serious depiction of the fighters as trophies as well as Master Hand itself being a trophy. Although never stated in the games most player refer to Master Hand as a male.